This invention relates to improved inverter apparatus.
Inverters capable of producing alternating currents of variable frequency and variable voltage are used for operating induction motors for driving electric motor cars, electric locomotives and railway cars which are fed from DC sources.
Variable frequency, variable voltage inverters can be classified into the following two types according to the method of varying the output voltage: (1) a variable DC voltage system and (2) a pulse width modulated system. According to a typical system of the former type the DC voltage is varied by effecting a gate control of an AC-DC converter and the output frequency of the inverter is varied by controlling the operating frequency of a DC-AC converter or inverter. According to the latter type, the DC output voltage from the AC-DC converter is maintained at a constant value whereas the output voltage and frequency of the inverter are varied by the inverter alone. Accordingly, in the former type, the variations in the output voltage and frequency of the inverter are performed in two steps whereas in the latter type such variations can be effected in only one step. Thus, the pulse width modulated type is suitable for rapid and precise control of the motor speed and is characterized in that there is no problem of shortage of the commutation voltage caused by the decrease in the DC voltage and that the power factor of the source is high. For this reason, the pulse width modulated type inverters increase their field of application which is aided by recent developments in improved control techniques.
This invention relates to improvements of the pulse width modulated type inverters.
When classified according to the output waveform, inverters are classified into (1) constant voltage type inverters and (2) constant current type inverters. As is well known in the art, in the constant voltage type inverter, the load side impedance as seen from the source side is low and since free wheeling diodes are connected in parallel oposition with the main semiconductor elements, the source side impedance as seen from the load side is also low. Usually an output of a rectangular wave form having a conduction width of 180.degree. is obtained.
The inverter of this type can be used as a pulse width modulated inverter as shown in FIG. 1 of the accompanying drawing in which 6 thyristors Qu, Qv, Qw, Qu, Qv and Qw are connected in a bridge circuit and free wheeling diodes Du, Dv, Dw, Du, Dv and Dw are connected in parallel opposition with respective thyristors. Terminals U, V, W of a three phase induction motor IM is connected to the AC output terminals of the inverter which is energized from a constant voltage DC source B. Since this type of the inverter is well known, the commutating circuit and the gate control circuit for the thyristors are not shown for the purpose of simplicity.
FIGS. 2(a) and 2(b) show the waveforms of the output voltage and output current of phase U where the output frequency of the inverter is 1/6 of the modulating frequency of the pulse used to perform the gate control of the thyristers. As can be noted from FIGS. 2a and 2b, as the output voltage and current contain a large pulsating components of high frequency, the iron loss, copper loss, efficiency and the torque of the motor decrease.
In the constant current inverter, since a large DC reactor is connected between the DC source and the inverter not only the load side impedance as seen from the source side but also the source side impedance as seen from the load side are high so that an output current of rectangular waveform having a conduction width of 120.degree. is obtained. This type of the inverter can not be used as a pulse wave modulated inverter.